Process for the production of peroxyacetic acid

ABSTRACT

PEROXYACETIC ACID IS PRODUCED BY A PROCESS WHEREIN AQUEOUS PEROXYDISULFATE IONS ARE GENERATED IN AN ELECTROLYTIC CELL AND REACTED WITH ACETIC ACID TO FORM AN AQUEOUS SOLUTION OF PEROXYACETIC ACID AND PEROXYDISULFATE REDUCTION PRODUCT. THE PEROXYACETIC ACID IS SEPA-   RATED FROM THE PEROXYDISULFATE REDUCTION PRODUCT AND THE LATTER CAN BE RECYCLED TO THE ELECTROLYTIC CELL FOR USE IN GENERATING NEW PEROXYDISULFATE IONS.

G. A. SERAD EVAL Filed June 2l, 1972 N.. mz: 7:55 2 e Iv A LK Iv v v o5:02a j e U E52 @1x1 Z238 292.152@ a 1 zo:u jmu \l\ Uzod AI N Aug. 13,1974 PROCESS Fon THE PRODUCTION oF lPERoxxmcETIc Acrn .T mz: lr/

United States Patent O U.S. Cl. 260-502 R 8 Claims ABSTRACT OF THEDISCLOSURE Peroxyacetic acid is produced by a process wherein aqueousperoxydisulfate ions are generated in an elecrolytic cell and reactedwith acetic acid to form an aqueous solution of peroxyacetic acid and aperoxydisulfate reduction product. The peroxyacetic acid is separatedfrom the peroxydisulfate reduction product and the latter can berecycled to the electrolytic cell for use in generating newperoxydisulfate ions.

This application is a continuation-impart of copending application Ser.No. 48,962, i'iled June 23, 1970, now abandoned.

'BACKGROUND OF THE INVENTION This invention relates to the production ofperoxyacetic acid by oxidation of acetic acid. More particularly, theinvention pertains to the use of peroxydisulfate ions as the oxidizingagent and to a continuous process for the preparation of peroxydisulfateions and peroxyacetic acid.

There are two principal methods heretofore known for the commercialproduction of peroxyacetic acid. iIn the first of these, hydrogenperoxide and sulfuric acid are reacted with acetic acid. Unfortunately,this method has several disadvantages. In particular, it requires theuse of very concentrated solutions of hydrogen peroxide (e.g., 90percent) which are expensive to ship and dangerous to handle. The secondprinciple method for the preparation of peroxyacetic acid involves the'vapor phase oxidation of acetaldehyde. This method requires largeamounts of reactants and is not adapted to the production of relativelysmall amounts of peroxyacetic acid.

Therefore, it is an object of the present invention to provide animproved process for the production of peroxyacetic acid.

Another object is to provide an improved process for the production ofperoxyacetic acid which involves relatively low shipment costs andreactants which are safe to handle.

Yet another object is to provide an improved process for production ofperoxyacetic acid which is not limited to the production of relativelylarge quantities of peroxyacetic acid.

These and other objects of the invention as well as the advantagesthereof can be had by reference to the following detailed descriptionand claims.

SUMMARY OF THE INVENTION An economical, self-contained method ofproducing peroxyacetic acid has been discovered which overcomes many ofthe above-mentioned problems previously associated with its preparation.The method of this invention involves the electrolytic formation ofperoxydisulfate ions by the anodic oxidation of sulfate ions in aqueousmedium. The peroxydisulfate ions are then reacted directly with aceticacid to form a peroxyacetic acid product which can be isolated byconventional methods. A byproduct of this reaction is a peroxydisulfatereduction 3,829,468 Patented Aug. 13, 1974 product which is returned tothe electrolytic cell for reoxidation and subsequent re-use. In apreferred mode of this invention (described hereinbelow) theperoxyacetic acid product is isolated in the form of an aqueous solutionby fractional distillation. The term peroxydisulfate reduction productas used herein is meant to include those products resulting from thereduction-hydrolysis of peroxydisulfuric acid and peroxydisulfate ions.Thus, when peroxydisulfate ions react with acetic acid in the presenceof water they are reduced to sulfate ions:

As this reaction proceeds, the peroxy-acetic acid is continuouslyremoved and equilibrium is shifted to the right. Likewise,peroxydisulfate (S2O8=) can be hydrolyzed to peroxymonosulfate (805:)which can be further hydrolyzed to sulfate ions (804:).

Thus, the term peroxydisulfate reduction product includes the aboveproducts, among others.

By utilizing the process of this invention, the costly and dangerous useof hydrogen peroxide is eliminated. Furthermore, the high ransportationcosts of raw materials are substantially reduced or eliminated, sincethe only materials continuously consumed in the process of thisinvention are electricity, water, and acetic acid. Moreover, the aceticacid recovered as a by-product from the use of peroxyacetic acid can berecycled and reoxidized, substantially reducing the need for a furtherreactant supply. Thus, the process of this invention, in cases whereacetic acid can be reclaimed, ideally only requires start-up materialsto charge the processing unit.

The peroxydisulfate ions suitable for use in the present process can beprepared by various methods including chemical oxidation of sulfuricacid. However, it is a feature of 'the present invention that theperoxydisulfate ions are prepared electrolytically.

The electrolytic cell useful in carrying out the preferredperoxydisulfate ion preparation process of this invention can be ofconventional design, Le., equipped with an anode and a cathode. However,due to the strongly oxidative nature of the products produced by theelectrolysis herein, certain modifications are desirable. Thus, thecathode is `advantageously constructed of carbon of graphite-containingmaterials. Furthermore, the anode should be of a material which is notsubstantially oxidizable. Examples of useful anode materials includeplatinum, lead oxide, nickel, nickel oxide, lead oxide coated carbon,gold, and the like. Preferably, the anode is formed of platinum.

The preferred electrolytic cell configuration comprises an internalanode surrounded by a membrane which in turn is further surrounded by-the cathode. This membrane can be formed of any of the common membranematerials such as those used in chlorine cells. Included yare asbestos,porcelain, ceramics and ion-permeable membranes. In a variation of thisconfiguration, a multichambered cell having alternating cathode andanode members separated by the above-described membrane can be used.This membrane is useful for inhibiting the undesirable electrolyticreduction of the peroxydisulfate ions. Thus, if peroxydisulfate ions areallowed to come into contact with the cathode, they would be immediatelyreduced back to the sulfate ion starting material.

The electrolytic cell -as described above is filled with an aqueoussulfate ion-containing solution. The concentration -of this solution canrange from about 1 molar to about 8-10 molar, preferably about 2-'5molar. Any suitable source of sulfate -ions can be used in thisinvention. However, it is preferred th-at the sulfate ion source =beselected from sulfuric acid, Water soluble metal sulfates, and watersoluble quaternary amine sulfates and the like, or mixtures thereof.Most preferred is a mixture of a sulfate salt such as ammonium sulfateand sulfuric acid, -these two constituents being present at the 0.25-3molar level and 1 5 molar level, respectively.

.Any convenient source of direct electrical current can be used in theprocess of the present invention. However, a control means such as arheostat should be attached to this source in order to regulate thevoltage applied to the cell itself. The applied voltage should be in therange of about 2.5 to about 10 volts. If less than about 2.5 volts 'areapplied, no appreciable formation of peroxydisulfate ions is possible.On the other hand, when the voltage is increased above the 10 volt levelthe current density (dened as the current per unit area available at theanode) Iincreases to such an extent that the cel'l begins to overheatthereby causing decomposition of the peroxydisulfate ions. Preferably,the electrolytic cell of this invention should be operated in the rangeof about 3-5 volts so that a current density at the anode of about 0.1to 5 amps/cm.2 results. The maximum current efficiency (defined as theamount of current which actually acts to produce peroxydisulfate ions)is attained with a current density of about 1 to 3 amps/cm?.

Normally, it is desirable that the temperature of the cell describedherein be maintained at the lowest possible level in order to obtainmaximum current efficiency. However, when acetic acid is present in theelectrolytic cell, as is the case whenever a continuous process isemployed, Athere is generally no significant relationship betweencurrent eiciency and temperature, especially when ambient or lowertemperatures are employed. However, in order to minimize the amount ofheat degradation of the peroxydisulfate ions, it is preferable tomaintain the electrolytic cell at a temperature of between about 20 C.and about 50 C.

When a continuous process is used according to the process of thepresent invention, unreacted acetic acid is charged to the electrolyticcell along with the peroxy disulfate reduction product (as definedabove). The presence of -this acetic acid has -a substantial effect oncurrent efficiency, and its concentration in the cell must therefore becontrolled. Acceptable current efficiencies (i.e., about 35%) areobtained when the concentration of acetic acid charged to the cell ismaintained at less than about 3 molar, and preferably less than about 2molar. Acetic acid concentration in the cell can be controlled bycontrolling (1) the amount of acetic acid introduced into the fractionaldistillation column, and (2) the efficiency and amount of reactionbetween peroxydisulfate ions and acetic acid to form peroxyacetic acid.Control of these factors is discussed below.

'In order to monitor the voltage and current at the anode describedabove, a reference cell is used. This cell is connected to the anode bymeans of a salt bridge opening at lone end in the immediate vicinity ofthe anode and connected at the other end to a conventional voltagemeasuring means. This bridge can contain any electrolyte, including thatwhich is present in the electrolytic cell (Le. a cellularelectrolyte).However, when non-cellular electrolytes are used, the end of the saltbridge opening into the cell anode area must be sealed with a4semi-permeable membrane so that none of the bridge electrolyte isallowed to co-mingle with the cell electrolyte.

The aqueous solution of peroxydisulfate ions prepared in theelectrolytic cell is pumped to a reactor where acetic acid and water, ifdesired, are added. After the reaction is completed, a solution ofperoxyacetic acid is separated from the resulting peroxydisulfatereduction product. 'I'his separation can be carried out by anyconventional method, including extraction, evaporation, or preferablydistillation. Acetic acid can be obtained as a raw material or it can beobtained from operations where it is formed as a by-product from thereduction of peroxyacetic acid. According to a preferred mode of thisinvention a fractional distillation column is used as both the reactorand as the means for separating the peroxyacetic acid product. In thisprocess, it is preferred that the aqueous peroxydisulfate ion solutionand the acetic acid enter the fractional distillation column near itsmiddle. At this point they are mixed and begin to react as follows:

The column itself can be of any standard design; however, for bestresults it should have about 10-50 theoretical plates (preferably about15-30) and a variable reflux ratio set at about 1/1 to 20/1 (preferablyabout 8/1-15/ 1). In addition, the distillation column should beequipped with a heated reboiler designed so that the liquid residencetime in both the column and the reboiler is in the range of about 10minutes to about 1 hour, preferably about 20 to 40 minutes.

As the reaction of the peroxydisulfate ions and the acetic acidproceeds, peroxyacetic acid is distilled and removed from the top of thecolumn in the form of its aqueous solution. Since water forms anazeotrope with peroxyacetic acid but not with acetic acid, the unreactedacetic acid is separated from the peroxyacetic acid-water azeotrope andremains in the reaction column.

In order to provide maximum reaction efficiency in the column, thetemperature at the take-olf point at the top of the column should bemaintained at about C.- C. preferably 110 C. to 115 C. This temperaturecan be controlled by varying the heat input to the reboiler located atthe bottom of the column. In a preferred arrangement, a temperaturesensor is imbedded in the column near the top take-off point. The sensoris adjusted -to control the temperature at this point at two or threedeg-rees above the desired take-olf temperature. When the temperature atthe sensor point increases above the desired temperature the sensor canthen act either to increase the reflux ratio of column or decrease theheat input on the reboiler. Likewise, when the temperature at the sensorpoint decreases below the desired level either the reux ratio can bedecreased or the reboiler temperature input increased.

While the peroxyacetic acid is being removed azeotropically, theunreacted acetic acid and the peroxydisulfate reduction products fallinto the reboiler where they can be continuously removed and returned tothe electrolytic cell along with any additional amounts of water lostdue to the azeotropic distillation of the aqueous peroxyacetic acidsolution. Thus, a continuous cycle resulting in the production of anaqueous solution of peroxyacetic acid can be formed, if desired. The useof this continuous cycle is the preferred process of this invention.However, the process of this invention need not be carried out as acontinuous one. For example, especially where highly etlicientproduction of peroxyacetic acid is required, the peroxydisulfatereduction products can be discarded or otherwise disposed of rather thanreturned to the electrolytic cell.

As can readily be seen several factors must be controlled in order toobtain maximum column and reaction efiiciency. The most obvious factorinfluencing reaction eiciency is the temperature at which the reactionis carried out. But this temperature is limited due to the distillationseparation methods employed herein. As previously stated, thistemperature should be maintained in the 110 C.-120 C. range. Residencetime in the column and reboiler is likewise a factor in determining theefficiency of the reaction. Again, this time factor should be in therange of about ten minutes to one hour. One of the most importantfactors affecting column efficiency is the concentration and ratio ofthe reactants. Generally, the

peroxydisulfate reaction mixture obtained from the electrolytic cellshould be at least 0.1 to 0.5 molar in peroxydisulfate ions. These ionscan be present at a level as high as about 4.0 to 6.0 molar. Moreover,the acetic acid concentration that is added to the column should be atabout the 2.0 to 6.0 molar concentration. When the process of thisinvention is in continuous operation a certain amount of acetic acidwill be present in the electrolytic cell due to its incompleteutilization in the distillation column and reboiler. Therefore, aportion of the peroxydisulfate solution will contain a concentration ofacetic acid. In order to maintain a continuous cyclic reaction theamount of acetic acid normally added should be reduced by the amountpresent in the peroxydisulfate solution. A final factor influencingreaction efficiency is the use when desired, of an appropriate reactioncatalyst. Preferred are the lower aliphatic sulfonic acids such asmethane or ethane sulfonic acid. This catalyst should, when present, beadded at about 0.25 to 2 molar level, preferably about the 0.5 to 1.5molar level.

By utilizing the above procedures and equipment, peroxydisulfuric acidor peroxydisulfate ions can be generated at about 50% current efiiciencylevel and reacted with acetic acid to produce about a 0.15 to 6.0 molaraqueous solution of peroxyacetic acid with a peroxydisulfate reactionefficiency of about to 35%, peroxydisulfate eiciency being defined asthe number of moles of peroxyacetic acid produced per mole ofperoxydisulfate 1on.

DESCRIPTION OF THE DRAWING The drawing depicts a schematic flow diagramcorresponding to the preferred process of this invention. In it, theperoxydisulfate ions are generated in the electrolytic cell (1). Theseions are then pumped down a column feed line (2) to the fractionaldistillate column (5). Concurrently with this feed, acetic acid ispumped through a feed line (3) and a valve (4) into the column (5). Asthe peroxyacetic acid is formed it is withdrawn through a take-olf line(6), and a take-olf valve (7). Unreacted acetic acid and theperoxydisulfate reduction products fall into reboiler (8) and afterfurther reaction are withdrawn through a return line (9) and a returnvalve (10). Additional water and peroxydisulfate ion to replace thatlost by the oxidation reaction and fractional distillation, is added tothe returning products through line (11), and and valve (12). Finally,all the returning products are pumped into the cell (1) for reoxidation.

The peroxyacetic acid solution prepared herein is a readily reactiveoxidizing agent. It is useful in the preparation of epoxide-containingmaterials as Well as in the bleaching of textiles and wood pulp.

EXAMPLES In the following examples, parts and percentages are by weightunless otherwise specified.

Example 1 An electrolytic cell was prepared using a 0.020 inch platinumwire coil having a 0.4 cm.2 surface area as the anode. Surrounding thisanode was placed a carbon 320 (Speer Carbon Co.) graphite cathode havinga surface area of 6.7 cm?. Into this cell immediately outside thecathode was placed a three-turn, 31/2 inch, glass cold water coolingcoil. Separating the anode from the cathode was a porcelainsemipermeable membrane. Power was supplied from a Kepco PBX-7-2C2-ampere power supply. This cell was filled with an aqueous solutionwhich was four molar in sulfuric acid, one molar in ammonium sulfate andone molar in methane sulfonic acid. Two one inch in diameter Oldershawcolumns were attached together to provide a total of about 20 trays andfurther attached to a reboiler. From a point about 15 trays above thereboiler the column was connected to the above cell through anelectrolytic reservoir. An acetic acid intake valve was provided at thissame point. Finally, the reboiler was connected to the above cellthrough a cell return pump and a water intake valve.

The electrolytic cell was activated and supplied with a current densityof one amp/ cm.2 at the anode and a voltage of about 4.5 volts. Byregulating the flow of cold water through the cell cooling coil, thetemperature of the cell electrolyte was maintained at a constant 25 C.As the peroxydisulfate ions were prepared in this cell they were pumpedinto the column at a 0.2 molar concentration and a flow rate of 6cc./minute. At the same time, acetic acid was introduced into the columnand maintained at a constant two molar level. The column was heated byapplying steam to the reboiler so that a temperature of 112 C. to 113 C.was maintained at the top-most portion of the column. Reux ratio wasadjusted to lO/l, producing thereby a column residence time of about 20minutes. Unreacted acetic acid and peroxydisulfate reduction productsare returned to the electrolytic cell and mixed with additional'water toreplace the water removed at the top of the distillation column. Usingthis continuous process, a 30 percent efficiency of peroxydisulfateconversion of acetic acid to peroxyacetic acid is obtained producing a0.43 molar (3.2 percent) aqueous solution of peroxyacetic acid at a rateof about one cc./ minute.

Example 2 A continuous peroxyacetic acid generator was assembled similarto that of Example l except that no methanesulfonic acid was employed.Peroxyacetic acid was formed as a 0.22 molar aqueous solution (l5percent efficiency) at a rate of about one cc./ minute.

While the instant invention has been described in terms of the examplesand description given above, it is to be understood that it is intendedto cover all changes and modifications which do not constitutedepartures from the spirit and scope of the invention.

What we claim is:

1. A process for the production of peroxyacetic acid comprising reactingacetic acid with a source of peroxydisulfate ions in an aqueous mediumat reflux, the concentration of peroxydisulfate ions being at least 0.1molar up to 6.0 molar and the concentration of acetic acid being about2.0 to 6.0 molar; and separating the peroxyacetic acid.

2. The process of claim 1 wherein the peroxyacetic acid is separatedfrom the reactants by fractional distillation at a temperature of about-1 15 C.

3. The process of claim 1, wherein the reaction is catalyzed by a loweraliphatic sulfonic acid.

4. The process of claim 1, wherein the peroxydisulfate ions aregenerated in an aqueous electrolytic cell comprising sulfate ions in aconcentration from about 1 to about 10 molar with an applied voltage ofabout 2.5 to about 10 volts and a temperature not above about 50 C.

S. A process for the continuous preparation of peroxyacetic acidcomprising (a) generating peroxydisulfate ions in an electrolytic cellcomprising an aqueous medium with a sulfate ion concentration from about1 to about 10 molar, and an applied voltage of about 2.5 to about 10volts, said cell being maintained at a temperature not above about 50C.;

(b) introducing the said peroxydisulfate ions to an aqueous reactionzone comprising acetic acid in a concentration of 2.0 to 6.0 molar toprovide a peroxydisulfate ion concentration of at least 0.1 molar up to6.0 molar and effecting the reaction of said ions with acetic acid atreux to form peroxyacetic acid and a peroxydisulfate reduction product;

(c) separating said peroxyacetic acid; and

(d) returning a peroxydisulfate reduction product stream to said cell,the proportion of acetic acid in such stream being maintained below 3molar.

6. The process of claim 5 wherein said reaction and separating ofperoxyacetic acid takes place in a fractional distillation systemwherein the product stream is removed as an aqueous azeotrope at about110-115" C.

7. The process of claim 5, wherein the source of sulfate ions in amixture of ammonium sulfate and sulfuric acid, the ammonium sulfatebeing present at the 0.25 to i;

3 molar level and the sulfuric acid being present at the 1 to 5 molarlevel.

8. The process of claim 5 wherein the applied voltage is 3 t0 5 voltsand the current density at the anode is about 0.1 to 5 amps/ cm?.

References Cited FOREIGN PATENTS 2,018,216 11/1971 Germany 204-79 HOWARDS. WILLIAMS, Primary Examiner R. L. ANDREWS, Assistant Examiner U.S. C1.X.R.

